Heat transfer in solar air duct with multi-V-ribbed absorber and grooved back-plate

The article presents an experimental study on heat transfer and friction behaviors in a solar air duct fitted with multiple V-shaped ribs on the absorber and delta-grooves on the back plate. Measurements were carried out in the test duct having a cross-section of width, W = 300 mm and height, H = 27 mm. The air flow rate inside the duct was varied to have Reynolds numbers based on the duct hydraulic diameter from about 7000 to 30,000. Two vortex flow devices: rib and groove turbulators, were introduced to generate the vortex flows along the duct. In the test duct, the upper/absorber plate having a constant heat-flux was mounted repeatedly by multiple V-shaped thin ribs with an attack angle (α) of 45° relative to main flow direction while the lower/back plate was grooved periodically in the delta/triangular shape with an attack angle (θ) of 60°. In the present investigation, the geometrical parameters of the ribs included three different rib- to duct-height ratios (e/H = BR = 0.108, 0.162 and 0.217) and three rib-pitch to duct-height ratio (P/H = PR = 1.0, 1.5 and 2.0). The experimental results have shown that the duct with the V-ribbed absorber at BR = 0.217, PR = 1 in conjunction with the delta-grooved back plate has the greatest heat transfer and pressure loss. However, the use of the combined devices with PR = 1, BR = 0.108 leads to the highest thermal performance and also provides greater heat transfer and thermal performance than employing the V-rib or the delta-groove alone.